Method and structure for gas tube modulation



May 21, 1957 w. M. WEBSTER, JR

METHOD AND STRUCTURE FOR GAS TUBE MODULATION 2 Sheets-Sheet l Filed Jan.26, 1953 .l 7, IM// May 21, 1957 w. M. WEBSTER, 'JR 2,793,313

METHOD AND STRUCTURE FOR GAS TUBE MoDULATIoN Filed Jan. 2e, 195s 2shetS-sheet 2 Lam/.47

INI/E N TOR.

y WzlljamMWebsZ United States Patent O METHOD AND STRUCTURE FOR GAS TUBEMODULATION William M. Webster, Jr., Princeton, N. I., assignor to RadioCorporation of America, a corporation of Delaware Application January26, 1953, Serial No. 333,257

9 Claims. (Cl. 313-189) This invention relates to gas discharge devicesthat have continuous grid control. More particularly it relates to suchgas discharge devices whi-ch have very high values of transconductanceand of lanode current with very low values of output impedance.

The type of gas` discharge devices with which this invention isconcerned are described in an article appearing in the IRE, volume 40,Number 6, June 1952, page 645, by L. Malter, E. O. Johnson, and W. M.Webster. These tubes, .or devices, have many advantages over the moreconventional types of gas tubes, however, these tubes also have certainlimitations.

One limitation upon these devices is that sensitivity of the 4Inodulating capabilities of `the control electrode is limited somewhat bythe fact that the control electrode normally must be capable of directlycontrolling ,a very Ierse magnitude ef Current- .Anether .limitationer1-these devieee .is that, even though the control elect-rode is anexcellent method ,of modulating the .Output eurrent ef .the .deviee ferSome ,instances it is desirable to have a plurality of modulating meansin the device One instance Where it ie desired to have a p1u relitv ,efmodulating means .in the deviee is fer mixing two or more incomingsignals.

,It ,is therefore en Object ef this .invention to provide a gasdischarge Kdevice of the .type under consideration hav ing a new andimproved means for modulating the output current.

It is another object `of this invention to provide a new and nevel sesdiseharce device ef the type under seaside-ration having a plurality lofmodulating means.

lt is Aa further object of this invention to provide a modulating meanshaving improved sensitivity `for gas discharge .devices ci the ,typeconsidered .by this invention.

These and other .objects are attained in accordance with the generalaspects of this invention by providing a gas discharge device comprisinga main cathode andan anode defining a main current path. The maincurrent path may, if desired, include a control electrode. An auxiliarygroup .of .electrodes is included within .the envelope that comprises anauxiliary lcathode that is sub stantially enclosed by an aperturedconstricting electrode. intermediate the auxiliary cathode and theconstricting electrode is arranged one or more probe type electrodes tomodulate the positive ion loss from within the constricting electrodeand thus modulate .the main current f low.

These and `other features and advantages will best be understood byreferring to the following .descriptions of the illustrated embodimentsvread `in connection 'with the accompanying drawings wherein likereference characters designate similar palts throughout the severalviews and in -Whiehi Figure 'l is a top sectional view through line1v1-1 lof Figure 2 of a gas discharge device constructed in accordancewith this invention;

Figure 2 is a .SeCQDl ViW hIOLlgh the line 2v2 Of Figure l;

Figure 3 is a schematic diagram of a circuit utilizing 2 a gas dischargedevice constructed in accordance with this invention;

Figure 4 is a graph of the current output versus the anode rpotentialfor diiferent values of probe potential showing the .characteristics ofa gas discharge device constructed in accordance with this invention;

Figure 5 is a graph of the output current versus the probe potentialshowing the transconductance of a. gas discharge device constructed inaccordance with this invention;

Figure 6 is a schematic diagram of a mixer circuit utilizing a gasdischarge constructed in accordance with this invention;

Figure 7 is a schematic diagram of another mixer circuit utilizing a gasdischarge device constructed in a cordance with this invention; l

Figure 8 is a top sectional view of another embodiment of a gasdischarge device constructed in accordance with this invention;

Figure y9 is a schematic circuit diagram of a modulator circuitutilizing a gas discharge device constructed in accordance with thisinvention; and

Figure l0 is a top sectional View of a further embodiment of a gasdischarge device constructed in accordance With this invention.

Referring now tothe drawings and Figures l and 2 in particular,y gasdischarge device 1 0 is illustrative of one form which this inventionlm-ay take and comprises a gas tight envelope 11 only partiallyindicated in Figure 2.

The device is provided with the usual stenl 12 through which the lead-inconductors are sealed as indicated. A main cathode 13 is supportedbetween upper and lower insulating supports such as micas 14 and 15. AU-shaped control grid 16 and a U-shaped anode 17 are also supportedbetween micas 14, 15 and, as shown, partially enclose cathode 13, Grid16 ycomprises a pair of U-shaped support members 18 and 19 between whicha plurality of parallel grid wires are supported in spaced relation.Grid 16 is provided with support members 21 which extend through micas14, 15 and serve to rigidly imount the grid. Anode 17 is made of sheetmetal andhas connected thereto support rods 22 which Vserve to rigidlylock the anode between vupper and lower ,micas 14 and 15, Opposite theopen ends of grid 16 and anode 17 is mounted a cylindrical constrictingelectrode 23 supported in a similar manner between micas 14 and 15 bymeans of support rods 24 connected thereto. The ends of the ycylindricalconstricting electrode lare closed by conductive end members 36 and 37as shown. An ionizing or auxiliary cathode 25 is mounted coaxially andconcentric with respect ,to `the cylindrical constricting electrode 23.The auxiliary cathode 25 is insulated from end members 36 an-d 37 bymeans of insulating material or by being spaced from the end members 3.6and 37 as shown. Closely spaced from auxiliary cathode 25 is a probeelectrode, or wire, 3 3. The probe electrode 38 is also insulated fromend members 36 yand 37 .either by means of some type lof insulatingmaterial or by being spaced from the end members as shown. The controlprovided by the probe electrode 38 decreases as the spacing between theauxiliary cathode y25 land probe velectrode 38 increases. It is for thisreason that the probeelectrode 38 is preferably closely spaced adjacentthe auxiliary cathode 25, i. e. approximately lor 2 mm.

As shown in Figures 1 and 2 constricting electrode 23 is provided withan elongated narrow slot 26, the center of which extends along a planepassing through the axis of probe electrode 3S, auxiliary cathode 25 andmain cathode 1 3. Ifhe size of slot 26 is approximately'l to 2 mm'Auxiliary .eafheete 25 ,is .connected te leeden 34 while the leads ofheater 2 8 are connected, respectively, to lead-ins 34 and 35. `Probeelectrode 38 is 'conw nected to lead-in 39 in the usual manner. One ofthe constricting electrode supporting rods 24 is connected to supportingconductor 31.

One of the grid support members 21 is connected to lead-in conductor 29which is sealed through stem 12. One of the anode support rods 22 isconnected to a supporting conductor 30. Both supporting conductors 30and 31 are sealed through stem 12 and serve as the main support membersbetween stem 12 and the electrode assembly. Lead-ins 32 and 33 areconnected to the leads .of heater 27 while the lead-in 32 is connectedto the sleeve of cathode 13 as shown.

Gas discharge device 10 is processed in the well known manner and isprovided with a gaseous atmosphere prior to' sealing off. Any suitablegas or mixture of gases may be utilized. The gas pressure will vary inaccordance with the specific envelope and electrode geometry andspacings. Furthermore, it is not believed that the gas pressure iscritical except that it is preferable to utilize a pressure which willfavor the formation of a selfsustaining ionizing discharge. In the tubenow being described as being illustrative of this invention helium wasused at a pressure of approximately l mm. or" mercury. However, anysuitable gas, preferably a noble gas, may be used with a pressure rangeof approximately .l to l mm. of mercury.

Referring now to Figure 3 for the operation of gas discharge devicethere is shown a schematic diagram of a circuit utilizing gas dischargedevice 10 constructed in accordance with this invention. to the positiveside of a variable source of potential 40. The negative side of source40 is connected through an ammeter 41 to main cathode 13. The maincathode 13 and grid 16 are connected to input terminals 42. Main'cathode 13 is also connected to the positive side of a potential source43. The negative side of source 43 is connected to auxiliary cathode 25through a current limiting resistor 44. Constricting electrode 23 isconnected to the positive side of a potential source 45. The other sideof source 45 is connected to auxiliary cathode 25 and to one of a pairof input terminals 46. Probe electrode 38 is connected to the other ofthe input terminals 46.

The sources of potential have been indicated as simple batteries but itshould be understood that any suitable potential sources may be used.The potentials are so selected that between auxiliary cathode 25 andmain cathode 13 a potential difference exists which is suticient tocause an ionizing discharge to occur therebetween. The potentialdifference between auxiliary cathode 25 and main cathode 13 (Vaux.) isapproximately 40 volts. Between the main cathode 13 and anode 17 isimpressed a potential (Vm.) that is less than the ionizing potential ofthe medium. The magnitude of (Vm.) may vary from approximately 2 voltsto 25 volts. The potential difference, i. e. source 45, betweenauxiliary cathode 25 and constricting electrode 23 is approximately 2volts. An input signal may be either to terminals 42 or to terminals 46.In the alternative, input signals may be applied to terminals 42 and 46.However, at this time it is assumed that no signal is being applied toVterminals 42 and thus grid 16 is electrically oating.

When the potentials, as set forth in the proceeding paragraph, areapplied to gas discharge device 10 an ionizing discharge occurs betweenauxiliary cathodeZS and the main group of electrodes through slot 26.Constricting electrode 26 serves to direct the ionizing, or auxiliary,discharge and also intensities the ionization thereof while utilizing avery small current of approximately 5 to l5 milli-amperes.- Theauxiliary discharge forms a plasma in device 10 that extends through thegrid openings and into the grid-anode region as well. Thus, there is ahighly conductive path from main cathode '13 through grid 16 to anode17. When the auxiliary Anode 17 is connected D discharge occurs,negative electrons emitted by the auxiliary cathode 25 move out ofconstricting electrode 26 toward the main electrodes. The rate of iongeneration in the main electrode group is dependent upon the rate ofelectron ow out of the region within the constricting electrode 23. Therate of electron ow out of constricting electrode 23 is dependent uponthe rate of positive ion flow into constricting electrode 23 'and thus,the plasma electron density is controlled by controlling the ion flowinto constricting electrode 23. For equilibrium in the device 10, therate of ion ow into constricting electrode 23 must equal the rate of ionloss from Within constricting electrode 23. Due to the positivepotential on constricting electrode 23, and end members 36 and 37, thepositive ions in the constricting electrode must go to auxiliary cathode25 or to probe electrode 38.

Under normal operating conditions the auxiliary cathode 25 is a prolificemitter of electrons and thus is space charge limited. Due to the factthat the auxiliary cathode 25 is space charge limited, the plasma isslightly negative, say .5 volt, with respect to the auxiliary cathode25. Because the plasma is negative with respect to the auxiliary cathode25, positive ions are repelled from the auxiliary cathode 25 thusleaving only the probe electrode 38, Within the constricting electrode23, which has an attractive force for the positive ions in this area.When a signal is applied to the probe electrode 38, the attractive forcefor the positive ions is varied in accordance with the signal and thusthe rate of ion loss within the constricting electrode 23 is varied. Ashas been explained, a variation of ion loss within the constrictingelectrode 23 varies the output or load current of device 10.

Referring now to Figure 4 there is shown a graph of the current outputversus the anode potential. This graph, which is plotted for differentvalues of probe electrode potential with respect to the auxiliarycathode potential shows that the characteristic of gas discharge device10 is pentode-like. The slope of the saturated part of thischaracteristic is equivalent to a resistance of about ohms but theproper load line depends upon the application of the device.

Referring to Figure 5 there is shown a graph of the output currentversus the probe electrode potential. The graph is plotted with aconstant anode potential of 6 volts positive with respect to the maincathode 13, 'and with a constant main cathode potential of 30 voltspositive with respect to the auxiliary cathode 25. The probe potentialis with respect to the auxiliary cathode. The input impedance used islow and is within the range of 150 to 800 ohms depending upon probepotential. The power gain as computed from these curves is approximately42 db. The transconductance is about 700,000 micromhos.

The internal operation of gas discharge device 10 is as follows: inregion 2 of Figure 5, as the probe electrode potential becomes morenegative, the ion current to it increases. In order to supply this ioncurrent, the operating arc drop, i. e. the auxiliary discharge voltagewithin the device, must increase. When the arc drop increases there isless voltage drop across the limiting resistor 44, of Fig. 3, andconsequently the auxiliary discharge current decreases. The decrease inauxiliary discharge current reflects itself in a decrease 'in main loadcurrent thus resulting in load current modulation. Conversely as theprobe Lelectrode potential becomes more positive, the auxiliarydischarge voltage decreases and the auxiliary discharge current and mainbond current increase.

Another method of modulating device 10, which gives a reverse slope tothe output current versus the probe electrode potential, i. e. anegative gm, is as follows: in region 1 of the graph of Figure 5 themain load current is increased by making the probe electrode potentialmore negative, which increases the auxiliary discharge voltage becausethe ionization eiciency improves while the auxiliary discharge currentisl essentially constant. Eventually, however, when the probe potentialreaches the dotted line in Fig. the auxiliary discharge voltage becomesan appreciable part of source 43 and the auxiliary discharge current andmain load current begin -to decrease with increased auxiliary dischargevoltage. This later phenomena accounts for region 2 of the graph.

Referring to Figure 6 there is shown a schematic circuit diagram for gasdischarge device 10 when used as a mixer. In this circuit the grid 16 isleft floating or could be connected to anode 17 or omitted. The anode 17is connected to main cathode 13 through the primary of an outputtransformer 47 and a source 49. The secondary of output transformer 47is connected to output terminals 48. Main cathode 13 is also connectedto the cathode of a vacuum tube 51 through an vauxiliary source 50. Atap on auxiliary source 50 is connected to constricting electrode 23.Auxiliary cathode 25 is connected to the anode of tube 51 and to oneside of the secondary of an input transformer S3. The other side of thesecondary of input transformer 53 is connected to probe electrode 38.The primary of input transformer 53 is connected to input terminal 54and the grid and cathode of tube 51 is connected to a second pair ofinput terminals 52.

In operation of the circuit shown in'Figure 6 a signal applied to inputterminals 54 modulates the main load current of device 10 as does asignal appliedy to input terminals 52. The two signals are mixed in thedevice and an output is taken lfrom terminals 48.

Figure 7 is a further mixer circuit diagram utilizing a gas dischargedevice constructed in accordance with this invention. This circuit issimilar to Figure 6 except that a constant bias 58 is applied betweenauxiliary cathode `25 and constricting electrode 23 and the secondsignal is applied to transformer 56 between the grid 16 and main cathode13. The other connections of this ligure are similar to those previouslydescribed so that further description is not deemed necessary at thistime.

In operation of the mixer circuit shown in Figure 7 a signal is appliedto input terminals 54 while a second signal is applied to inputterminals 57. These two input signals both modulate the main loadcurrent of device 10 and thus mix the signals.

Referring now to Figure 8 there is shown a top sectional view of apreferred embodiment of this invention utilizing a pair of probeelectrodes 65 and 65 one adjacent to each side of aperture 66 inconstricting electrode 67. Auxiliary and main cathodes 68 and 69respectively as well as anode 70 are similar to those previouslydescribed therefore further description of these electrodes is notdeemed necessary. If two means for modulating the main load current aredesired a control grid (not shown) may be used in the main current pathas shown in Figure 9, or in the alternative a different signal may beapplied to each `of the probe electrodes 65 and 65. However, it ispreferred to apply the same signal to both probe electrodes 65 and 65'so that the one signal will constriction modulate the auxiliarydischarge as well as modulate the ion density inside constrictionelectrode 67. It has been found that when the probe electrodes 65 and65' bracket the aperture 66 the control obtained from the probes is at amaximum in sensitivity.

Referring now to Figure 9 there is shown a schematic diagram andrepresentation of an amplier circuit utilizing the gas discharge deviceshown in Figure 8. In the circuit the anode 70 is connected through aload 71 to the positive side of source 72. The negative side of apotential source 72 is connected to main cathode 69 and to the positiveside of a potential source 73. The negative side of source 73 isgrounded as is auxiliary cathode 68 and one side of the secondary ofinput transformer- 75. A tap on source 73 is connected to constrictingmember 67. Probes 65 and 65 lare connected to the other side of thesecondary of input transformer 75. The primary of input transformer 75is connected to input terminals 74.

Referring now to Figure l0 there is shown a top sectional view of a gasdischarge device constructed ac. cordance with this. invention that isespecially adapted to operate as a mixing device. The electrodestructure of this device includesa main cathode 77 and a main anode 76.The auxiliary group of electrodes includes an auxiliary cathodesurrounded by a constricting electrode 78 having an aperture 79 therein.Intermediate cathode 80 and constricting electrode 78 and on each sideof aperture 79, i. e. bracketing aperture 79, is a pair of probeelectrodes 82 and 82', Closely spaced from auxiliary cathode 80, andpreferably on the side thereof away from aperture 79 is still anotherprobe electrode 81.

The preferred form of operation of this device is to connect probeelectrodes 82 and 82 together and apply a signal thereto. This signalmodulates the main anode current as has been described. A second signalis applied to probe electrode 81 which also modulates ythe main anodecurrent and thus mixing occurs.

While I have indicated several embodiments of my invention of which I amnow aware and have also indicated several speciiic applications forwhich my invention may be employed, it will be apparent that myinvention is by no means limited to the exact forms illustrated or theexact uses indicated, but that many variations -may be made in theparticular structure used and the purpose for which it isemployedwithout departing from the scope of my invention as set forth in theappended claims.

I claim:

l. A gas discharge device, comprising a sealed en ve lope having anionizable gaseous medium therein, a main thermionic cathode yand ananode defining a main current path within said envelope, means includingan auxiliary thermionic cathode surrounded by an apertured constrictingelectrode within said envelope for producing a plasma including positiveions within a region including said constricting electrode and said maincurrent path, and electrode means within said constricting electrode formodulating the density of positive ions in said plasma whereby thecurrent in said main current path is modulated.

2. A gas discharge device, comprising a sealed envelope having anionizable gaseous medium therein, a main cathode and an anode defining amain current path Within said envelope, means including an auxiliarythermionic cathode surrounded by an apertured constricting electrodewithin said envelope for producing an auxiliary discharge resulting in aplasma throughout a region including said constricting electrode andsaid main current path, and electrode means within said constrictingelectrode for constricting said auxiliary discharge and for modulatingthe density of said plasma whereby the current in said main path ismodulated.

3. A gas discharge device, comprising a sealed envelope having anionizable gaseous medium therein, a main cathode and an anode arrangedin spaced relationship within said envelope, -an auxiliary cathode forproducing a plasma within a region of said device, an aperturedconstricting electrode surrounding said auxiliary cathode but insulatedtherefrom and within said region, said constricting electrode beingspaced from said main cathode and from said anode within said envelope,and electrode means within said constricting electrode and insulatedtherefrom for modulating the ion density of said plasma formed by adischarge from said `auxiliary cathode.

4. A gas discharge device, comprising a sealed envelope having anionizable gaseous medium therein, a main thermionic cathode and an anodearranged in spaced relationship within said envelope, an auxiliarythermionic cathode for producing a plasma within a region of saidenvelope, an apertured constricting electrode surrounding said auxiliarycathode but insulated therefrom and within said region, saidconstricting electrode being spaced from said main cathode and from saidanode within said envelope, a pair of probe electrodes within saidconstricting electrode one spaced on each side of the aperture in said 7constricting electrode and insulated therefrom, and at least one otherprobe electrode within said constricting electrode but insulatedtherefrom and spaced adjacent said auxiliary cathode.

5. A gas discharge device, comprising a sealed envelope having anionizable gaseous medium therein, a main thermionic cathode and an anodearranged in spaced relationship within said envelope, an auxiliarythermionic cathode for producing `a plasma within a region of saidenvelope, a hollow apertured constricting electrode surrounding saidauxiliary cathode but insulated therefrom and within said region, saidconstricting electrode being spaced from said main vcathode and fromsaid anode within said envelope, and a pair of probe electrodes withinlsaid constricting electrode one spaced on each side of the aperture insaid constrictingelectrode and insulated from said constrictingelectrode for modulating the density of said plasma.

6. A gas discharge device, comprising a sealed envelope having anionizable gaseous medium therein, a main thermionic cathode and an anodearranged in spaced relation within said envelope, an apertured tubularelectrode having its ends closed spaced from said main cathode and saidanode, an auxiliary thermionic cathode within said apertured electrodeand spaced therefrom, and at least one probe electrode within saidapertured electrode spaced from said auxiliary cathode and insulatedfrom said apertured electrode.

7. A gas discharge device, comprising a sealed envelope having anionizable gaseous medium therein, a main thermionic cathode and an anodein spaced relationship within said envelope, an apertured hollow tubularelectrode having its ends closed spaced from said main cathode and saidanode, an auxiliary therrnionic cathode within said apertured electrodeand spaced therefrom, and a probe electrode within said aperturedelectrode spaced from said auxiliary cathode and insulatedfrom saidapertured electrode.

8. A gas discharge device, comprising a sealed envelope having anionizable gaseous medium therein, a main thermionic `cathode and ananode in spaced relationship within said envelope, an apertured hollowtubular electrode having its ends closed spaced from said main cathodeand said anode, an auxiliary thermionic cathode within said aperturedelectrode and spaced therefrom, and a pair of probe electrodes withinsaid apertured electrode one spaced on each side of the aperture in saidapertured electrode, said probe electrodes being spaced from saidauxiliary cathode and from said apertured electrode.

9. A gas discharge device, comprising a sealed envelope having anionizable gaseous medium therein, a main thermionic cathode and an anodedefining a main current path within said envelope, means including anauxiliary thermionic cathode surrounded by an apertured constrictingelectrode for producing a plasma within a region of said deviceincluding said constricting electrode, and electrode means within saidconstricting electrode for density modulating said plasma whereby thecurrent in said main current path is modulated.

References Cited inthe file of this patent UNTED STATES PATENTS1,917,739 Schroter July 11, 1933 2,443,407 Wales June 15, 1948 2,445,679Lemmers July 20, 1948 2,611,880 Webster Sept. 23, 1952

